It depends on the plate, typically a plate will indicate whether it is standalone (meaning it needs no soft armour to work at its rated threat level) or if it requires soft armour as well to work at that rated threat level. The ceramic plates currently for sale probably wouldn't benefit too much from the added soft armour, but the steel plates on the other hand might for the reason that spall is typically an issue with steel plates. Although steel plates may easily stop the rated threat, the spall ricocheting off the plate may still be fatal. If your soft armour is inserted in-front of the plate, it would be able to capture that spall. So if you're purchasing steel plates and the soft armour is inserted in-front of the plate, there may be a benefit. However, the ceramic and UHMWPE plates probably won't get much of a benefit from the addition of soft armour.